when i connect it, it shows up on desktop and in finder, but when i try drag and drop anything small or big, it wont allow me to put anything on to the drive, usually a green plus sign shows up as you know, but now a circle with a line through it,

Go to System Preferences > Time Machine, and select a backup disk then try running it again. You can't backup on a NTFS or FAT32 drive, it should be a Mac one (Mac OS Extended Journal) I think. If it's NTFS or FAT32 and you'd like to convert, run Disk Utility (found in the Applications > Utilities folder) and do it.